Interspecific variations in leaf litter decomposition and nutrient release from tropical mangroves

Efficient nutrient cycling through decomposition of leaf litter often regulates the high productivity and subsequent carbon sequestration of mangrove ecosystems along the land-ocean boundary. To understand the characteristics and the potentials of mangrove leaf litter in supplying organic carbon and nutrients to the coastal waters, four major mangrove species (A. officinalis, R. mucronata, H. littoralis and S. apetala) of Bhitarkanika mangrove forest, Odisha, India, were examined in controlled environmental conditions. Half-life time (t₀.₅), estimated for decomposition of those mangrove leaf litter materials ranged from 18 to 52 days. During the incubation experiment, organic carbon from mangrove leaf litter was released primarily through physical processes and was available for heterotrophic respiration. Among the four species, leaf litter of S. apetala with the lowest initial C/N ratios, released organic carbon with low molecular weight (labile substances) that has a relatively higher potential to support the aquatic food web. On the contrary, leaf litter of R. mucronata released organic material with relatively higher molecular weight (humic substances, higher aromaticity), which revealed its superior non-labile characteristics in this unique environment. The mean total heterotrophic bacterial (THB) population in the incubation was around nine-fold higher than the control. THB population growth and Chromophoric Dissolved Organic Matter (CDOM) spectral data further suggested the rapid release of highly labile and recalcitrant carbon from S. apetala and R. mucronata (between 7th and 21st day of incubation), respectively. The mean litter fall from the Bhitarkanika mangrove forest was estimated to be 11.32 ± 1.57 Mg ha^-1 y^-1 and its corresponding carbon content was 5.43 ± 0.75 Mg C ha^-1. The study revealed the role of leaf litter leachates as an important food source to microbial communities in the adjacent coastal waters, in addition to a potential carbon sequesterer through long-term burial in mangrove soil and export to the deep sea.